1. Field of the Invention
The present invention relates to a pixel circuit using a light emitting display device element, a light emitting display device and a driving method thereof. The present invention particularly relates to a pixel circuit configured by an organic light emitting diode (Organic Light Emitting Diode, hereinafter called OLED) element and a drive circuit for supplying a current to the OLED element, a light emitting display device including the pixel circuits in a matrix form and a driving method thereof.
2. Description of the Related Art
In recent years, the research and development of OLED displays using organic light emitting diodes (OLED) as light emitting elements has been underway. In the OLED displays, an active-matrix (Active-Matrix, hereinafter called AM) type OLED display configured by pixel circuits including OLED elements and pixel circuits including circuits for driving the OLED elements is commonly used. AM type OLED displays elongate the service lives of the OLED elements, suppress power consumption, and can realize high image quality. The pixel circuit includes a thin-film transistor (Thin-Film-Transistor, hereinafter called TFT) as a component. A substrate and a TFT portion of the OLED display are mainly called a back plane.
As semiconductor materials of the TFT of the back plane for an AM type OLED display, amorphous silicone (amorphous-Si, hereinafter called a-Si), and a polysilicon (Poly-crystal-Si, hereinafter called p-Si) and the like are studied. Further, a TFT (hereinafter, called AOSTFT) which uses a thin film of an anamorphous oxide semiconductor (amorphous-oxide-semiconductor, hereinafter called AOS) as a channel layer of the TFT has been newly proposed.
As AOS materials, there are cited, for example, an amorphous oxide (amorphous-In—Ga—Zn—O, hereinafter called a-IGZO) of indium (In), gallium (Ga) and zinc (Zn), and an amorphous oxide (amorphous-Zn—In—O, hereinafter called a-ZIO) of zinc (Zn) and indium (In). AOS TFT includes mobility ten times as high as or more of the TFT (hereinafter called a-Si TFT) having a-Si as a channel layer, and is considered to obtain high uniformity due to amorphousness. Accordingly, these TFTs are promising as the TFTs of the back planes for displays. Nomura et. al., Nature, vol. 432, pp. 488-492, 2004 and Yabuta et. al., APL, 89, 112123, 2006 describe TFTs using a-IGZO.
Meanwhile, the pixel circuits including functions of correcting characteristic change and variation are studied, because of characteristic change by electrical and thermal stress in an a-Si TFT and an AOS TFT, and because of characteristic variation due to grain boundary in a TFT using p-Si as a channel layer (hereinafter, called p-Si TFT). These pixel circuits are broadly divided into those based on two techniques that are a current-writing type which determines current capability of the TFT controlling the current to be supplied to OLED elements by a current provided from outside the pixel circuit, and a voltage-writing type which determines the current capability of the TFT by applying a voltage.
In a current-writing type pixel circuit, the voltage of the TFT is determined by the current which is applied, and therefore, the current which is supplied to the OLEDs can be controlled irrespective of the threshold voltage expressing the characteristic of the TFT, and the value of mobility. Meanwhile, in the voltage-writing type pixel circuit, the current of the TFT is determined by the voltage which is applied, and therefore, a current with the threshold voltage being corrected and the mobility being uncorrected is supplied to OLEDs. Therefore, a current-writing type pixel circuit can be generally said to be able to control the current to be supplied to OLEDs with higher precision.
However, in the case of a current-writing type pixel circuit, a line load on the display is charged and discharged with a current, and therefore, much time is taken for writing. Accordingly, the current-writing type pixel circuit is difficult to be applied to a large screen display, because as the display size is larger, the line load becomes larger. Therefore, as described in Lee et. al., IEEE Transaction of Electron Devices, vol. 54, 2403, 2007, application of a current-writing type pixel circuit to a large screen display by providing a unit for decreasing the current for driving the OLED elements for a pixel circuit as compared with the write current is being studied.
The pixel circuit described in Lee et. al., IEEE Transaction of Electron Devices, vol. 54. 2403, 2007 includes two capacitor elements. This pixel circuit supplies to the OELD elements a lower current as compared with that at a current write time by using the gate voltage of the driving TFT determined by the current at the time of current write decreasing by the charge pump effect if the voltage at one terminal of one capacitor element is decreased when the OLED elements are driven.
In order to realize display with high quality with an AM type OLED display, it is required to correct difference in the characteristics of the component elements such as the change with time of the voltage-luminance characteristic of the OLED elements, the characteristic variation of the TFT which is the component of the drive circuit, and the TFT characteristic change due to electrical stress. Further, especially in a large screen display, writing of current takes much time, and application of a current-writing type pixel circuit with high precision is difficult.
An object of the present invention is to provide a light emitting display device and a driving method thereof which solve the aforementioned problem by a configuration and a driving method which are simpler than the pixel circuit described in Lee et. al., IEEE Transaction of Electron Devices, vol. 54, 2403, 2007.